Screw type fluid propelling apparatus



1940- J. T. M INTYRE 2 5 I SCREW TYPE FLUID PROPELLING APPARATUS Filed Sept. 25, 1938 2 Sheets-Sheet 1 //0 INVENTOR J. 1' Me INTYREH' fies. 1% 39m J. "r. M RNTYRE 2,224,519

- SCREW TYPE FLUID PROPELLING APPARATUS Filed Sept, 23, 1958 2 SheetsSheet 2 J. T. Mc INTYRE 7 BY M) f 1,8)? ATTYS.

INVENTOR Patented Dec. ii), 1940 UNITED SCREW TYPE FLUID PROPELLING APPARATUS Application September 23, 1938, Serial No. 231,428 In Great Britain March 5, 1938 20 Claims.

This invention relates to screw type fluid pro polling apparatus and particularly to such apparatus including an enclosing casing; and is a continuation in part of my prior application Serial No. 195,629 filed on March 12, 1938, and

my prior application Serial No. 195,630 filed on' March 12, 1938.

In this specification, "monometric efliciencyf means the ratio of the pressure actually obtained, to the theoretical pressure corresponding to the impeller tip speed; and volumetric efiiciency" means the ratio of the rate of flow of fluid, to the square of the impeller radius multiplied by the impeller tip speed. The term geometric pitc is used in its ordinary sen'se oi the axial length of one turn of the helix formed by helical extension of the chord of a section of an impeller blade or guide vane; but its use is restricted to discussion of difierent geometric pitches at the varying radii of a blade or vane. A helix is either right-hand or left-handed accordingly as it is generated by clockwise or by counterclockwise rotation of its generating radius moving forward along its axis; and the term helical han is used herein for comparing-An this resp'ectdifierent helical elements or formations, or elements or formations when considered as portions of helices. The use or the words relative curvature" is restricted to the description of impeller blades to denote the ratio of the maximum ordinate between the median camber line of an aerofoil element and-the chord joining the two ends of said median camber line, to the length of this chord.

Concavity as discussed in this specification has reference only to the relationship of the concave surface of a blade or vane to pure circumferential direction, for the purpose of specifying whether said concave surface faces, in that limited respect, in the same way as, or in the opposite way to, the sense of rotation of the impeller. Angularity of the chord of the concave suriace to said circumferential direction or to the axis of the machine is absent from this meaning of the term. The concavity of the impeller blades faces in the same way as the sense of rotation of the impeller.

The object of the invention is to provide a fluid propelling machine of the kind in question whereby high manometric efticiency can be attained with good volumetric and mechanical eiflciencies; mechanical efl'lciency being the ratio of total power in the delivered fluid to the power input to the impeller shaft.

The feature of a screw type machine which most considerably affects its manometric efliciency is, as is well known, the lower linear speed of the impeller blade sections towards the hub as compared with those towards the tip; with the consequence that, in order to avoid recircuiatory d turbulence and radial flow components in the stream of fluid outgoing from the impeller, re-

. sulting in low volumetric and mechanical eificiencies, it is necessary so to restrict the pressure building capacity of the blade sections in ii the tip zone as to equalize itsubstantially with the limited pressure building capacity of the blade sections in the hub zone.

The problem of equalizing the pressure obtainable at different radial sections of the impeller blades has received a considerable amount of attention and the manometric efficiency of screw apparatus has consequently been progressively improved. The present invention not only enables a high degree of equalization to be attained, but also enables the manometric efliciencies of all blade sections to be increased very materially in comparison with apparatus heretofore known.

According to the invention a screw type fluid propelling apparatus comprises an impeller hav ing blades of aerofoil cross section increasing in width and in total area. progressively in the direction from tip to hub with conforming relative curvature, the geometric pitch of the blades progressively decreasing in the said direction, a combination with fluid inlet means comprising a helical guide vane assembly co-axial with the impeller and arranged to impart counterswirl to the fluid ingoing to the impeller, said vane assembly being of decreasing geometric pitch in the direction from tip to hub.

Screw fans and propellers constructed accord-- ing to the invention are illustrated in the accompanying drawings in which:

Figure l is a longitudinal section of a single stage fan.

Figure 2 is a developed plan of a portion of the inlet guide vane assembly, the impeller, and the outlet guide vane assembly of Figure 1.

Figures 3 and 4 are simplified plans of the sim ilar parts of two-stage fans.

Figure 5 is a fluid velocity diagram corresponding to Figure 2.

Figure 6 is a pressure diagram.

Figure 7 is a diagram in which blade and inlet guide vane cross sections are shown correlated to the radii of the blade elevation at which they occur and so as to exhibit their geometric pitches.

Figure 8 is a graph showing geometric pitch variation of the blade.

craft.

In the drawings 2 indicates the impeller hub,

.3 impeller blades, 4 inlet guide vanes or assemblies thereof, and 5 outlet guide vanes orassem blies thereof. In Figures 3 and 4 interstage guide vanes are indicated by 6 and 6s. The general direction of fluid flow through the apparatus is parallel with the axis I, and in the illustrated example'its sense is assumed to be left to right, as indicated by arrows G. The sense of the rotation of the impeller or impellers is clockwise when viewed from the left and is indicated in some of the figures by arrows H.

Referring more particularly to Figures 1 and 2, the impeller is carried upon a driving shaft 8 mounted in bearings 9 and positioned axially within the casing Ill. The latter also encloses the inlet guide vanes 4 and the outlet guide vanes I, if present; and is substantially cylindrical about said parts in order to keep the flow free from radial components.

The function of the inlet guide vanes 4 is to impart to the fluid entering the impeller blades a movement component which is circumferential and contrary in sense to the circumferential movement of the blades; that is, in the illustrated example, an anti-clockwise circumferential movement component. This movement component is referred to as counterswirl. The vanes may be assisted in performing this function by other means for changing fluid motion direction; but in all cases the inlet guide vanes 4 are arranged to determine the final direction of the fluid when it is. delivered to the impeller. In order to produce the counterswirl the guide vanes 4 are helical and co-axial with the impeller; of the same helical hand as the impeller; and their concave surfaces face against the concave sur-= faces ofthelimpeller blades. They are preferably of aerofoil cross section in order to promote streamline flow. Moreover they usually decrease in width in the direction from tip to hub.

The proposal has been made to provide a screw impeller with inlet guide vanes which deliver the fluid to the impeller with a counterswirl sufllclent to counter-balance the impeller drag and so to cause the fluid to leave the impeller with pure axial movement; but in the present invention counterswirl is employed ,as an essential factor in pressure building, without any necessary magnitude relationship between it and the drag, and in combination with an impeller which is structurally capable of producing and utilizing a counterswirl of such magnitude and so differentiated radially as to attain in practice the pressure rise latent in the counterswirl principle.

This principle may be explained with reference to Figures 5 and 6 which are respectively velocity and pressure diagrams of some arbitrarily chosen annular element of the fluid stream.

Let Va'represent in magnitude and direction, the axial velocity set up, by the impeller, in the fluid flowing to it. The guide vanes 4 divert the flow into direction Vi, thus tending to add to the axial component Va a circumferential component represented in magnitude by Vwa and opposite in sense to the sense of rotation of the impeller. Rotation of the impeller however tends to set up in the'fluld the circumferential drag velocity represented by d, in the same sense as such rotation. The eifective circumferential component of the velocity of the incoming fluid is therefore the algebraic sum of Vwa and d, represented by Vwe.

Pva indicates the absolute linear velocity of the V by addition of Vwe, so does the pressure set up by the impeller increase in a'greater proportion.

Figure 6 illustrates the corresponding pressure variations. Fluid entering the guide vanes 4 with a pressure p loses some of said pressure due to its acceleration by the guide vanes and enters the impeller with pressure in. Being then acted upon by the impeller blades, its pressure is raised to the value represented by m.

However, velocity and pressure changes of the kind indicated by the above mentioned diagrams cannot be obtained in magnitude sufllcient 'to build up pressures greater than those producedby fans of known design, or alternatively sum- 'cient to obtain heretofore usual pressures with less angular impeller speed, unless the impeller blade sections at the various radii are made adequately powerful.

Accordingly, the impeller blades are made throughout their length of a form known per se, viz., of aerofoil section, by which is meant a section defined by back and front surfaces so differently cambered that the section is curved in the direction of the blade width and so also as to.

make the section of substantial thickness relatively to its chord length and. with its leading edge thicker than its trailing edge; such section progressively increasing, in the direction from tip to hub, in its structural thrust-producing factors, vlz., its blade section width, total area, and preferably also the ratio of thickness to chord length, and the relative curvature at all sections being in conformity with the varying values of said factors.

Such increase is shown in Figure '7 .where A, B, C represent the cross section of the blade at three different radii, viz the tip radius m, an intermediate radius rb and the hub radius rc respectively. It will be seen that the chords of the aerofoil sections, indicated at the different radii by ca. .cb, cc, increase in length progressively from section A to section C and that the ratio of thickness t to chord length, that is, the relative thickness of the aerofoil section, also progressively increases so that the whole blade section area increases in the same direction. For convenience of exhibiting this feature as well as the geometric pitch variation hereafter described, the leading ends of the chords of A, B, C are shown as assembled on a radial line! I, and not in their true positions relatively to one another. The relative curvature of the thus varying sections is adjusted in the manner well known in aerofoil dmigning, to ensure that the factors mentioned co-operate to give the required lift or thrust combined with the op imum drag/lift ratios for the various sections of the blade. In practice the relative curvature remains constant; or, as is shown, increases in the direction from tip to hub,

'A further characteristic of the impeller blade is that its geometric pitch decreases in the direction from tip to hub. This is shown by the prolongation oi the chord lines ca, ab and cc to intersect the impeller axis 1, so that the distance of each intersection from the common radial line II is a measure 01. the geometric pitch. These distances are indicated by PA, PB and PC respectively and it'will be observed that they dimin ish in magnitude according to the radial distance of the relevant section from the axis I. The same values are shown in Figure 8 plotted as a graph M or which the ordinate values are geometric pitch and the abscissae are fractions of the 1mpeller radius. The rate of decrease is usually non-linear, as indicated by the curving nature of the graph M.

The inlet guide vanes 4 similarly decrease in geometric pitch in the direction from tip to hub;

as will be apparent mm the directions of their chord lines Kc, Kb, Kc which, it continued, would intersect the axis I at diflerent, points, progressively closer to line H. Such progressive decrease oi the geometric pitch of the guide vanes gives rise to a corresponding increase oi the counterswirl velocity relatively to the axial vel'oclty component and is thus a material factor in diminishing the discrepancy between the pressure building capacity at the tip zone and the hub zone of the impeller. The diminution of geometric pitch in the blades and the vanes respec- I tively is such as to maintain the best angle of attack a at all radii.

It is the case that the steepening of the blade sections towards the hub due to the decreasing geometric pitch, tends to increase the drag in the hub zone, and so diminish the amount of energy available to be put into pressure energy; yet the facility with which pressure energy can be obtained by means or the counterswirl renders such tendency of secondary importance. An equivalent pressure increase in the hub region cannot be obtained by increasing the impeller blade geometric pitch towards the hub, because such increase, similarly to the decrease of the present invention, is accompanied by increase of drag, but at a disproportionate rate which gives rise to centrifugally driven radial flow components in the direction from hub to tip of considerable magnitude. The decreasing geometric pitch in the hub region has the incidental advantage of enabling the necessary wide blade section at the hub to be accommodated on a hub which is unusually short in the axial direction and for that reason structurallyadvantageous.

,The ccunterswirl may be applied only to the cgntral area of thestream near to the hub where the loss of pressure building capacity is greatest; or to the whole area swept out by the blades.

Also its magnitude in relation to impeller speed...

may be so selected as to meet various conditions. The principal object of the present invention being the attainment of high pressures, the velocity of the counterswirl is preferably arranged to. be in excess of the drag at all radial distances. This is demonstrated in Figure 5 where @ecircumfenential velocity component Vwa (that is the added counterswirl) is considerably greater than the drag velocity d and consequently their algebraic sum, Vwe, is a velocity oi substantial magnitude contrary in sense to the velocity of the blades. This results in a high value of Q and so of VT; and the production by the impeller of high fluid pressures as above described. It is believed that the production oihigh pressures by the aid of counterswirl in excess of drag has not heretotore been attempted. -By means of it and the other characteristics of the invention a cased screw fan can be built which produces pressures comparable with those of centrifugal fans, for instance with manometric emciencies ranging from 0.3 to 0.5 and more; while having considerably greater mechanical emciency than a centrifugal fan.

when Vwa is greater than d the fluid leaves the impeller stage with the velocity represented in magnitude and direction by Va and comprisinga residual counterswirl component Vwe. According to the invention provision is made at the outlet end of the apparatus for converting said residual counterswirl Vwe into axial movement, with gain of pressure, by means of a helical guide vane assembly 5 co-axial with the impeller. Said assembly diflers from any outlet guide vane assembly known to the inventor in two respects.

Firstly its helical hand-or more particularly the helical hand of the tangent ll (Figure 5) to the median line at the receiving ends of its vanesis the same as that of the chords of the impeller blades (and in the present case, as that of the inlet vane assembly 4). Secondly its concavity faces with the concavity of the impeller blades. A further characteristic of the assembly 5 is that the said tangent it makes a substantial angle, b

Figure 5, with the axis 7 so as to receive without shock the residual counterswirl from the impeller. The kinetic energy thus suppressedby the assembly 5 appears as. additional pressure energy. which is indicated in Figure 6' by the value p In the case of multi-stage machines, such as are illustrated in Figures 3 and 4, there is provided, between each pair of consecutive impeller stages, an interstage guide vane assembly 6 or 8:: which serves as the outlet guide tor the preceding impeller stage 2, 3, and as the inlet guide for the succeeding impeller stage 2a, 3a; and is adapted to add counterswirl to the fluid about to enter the second impeller 20, 3a.

In the case mentioned in the last paragraph but one, where a counterswirl component Vwe is present in the velocity of the fluid leaving the first impeller 2, 3, the assembly 6 (Figure 3) is of b with the axis i. They thus receive fluid with absolute velocity Va and deliver it with an abso liitevelocity which, in most cases, is similar-in direction and magnitude to Vi. ,Thus the manometric efliclency of the second and subsequent stages is increased in the same way as that of the first stage, the pressure building being cumulative as shown in Figure 6. In this figure the dotted line shows drop of pressure-from p to 12 due to the interstage guide vanes 6, increase from p to p due to the second impeller 2a, 3a, and further increase to 10 due to the final outlet guide vanes 5."

The outlet vanes 5 or the intermediate vanes 6 do not necessarily follow any rule as regards their geometric pitch at diiierent radii; although this respect is accompanied by substantial de-.

crease of mechanical efliciency due inter alia to the increased frictional resistance at the large hub surface; and moreover the volumetric emciency is low. The present invention enables high pressures to be attained with reasonable hub to tip diameter ratios; say one-half.

Figure 4 illustrates a construction suitable for attaining unusual manometric eiflciency when a 5 high value of volumetric efliciency is also essential, for instance in a case where the average geometric pitch must be large in relation to the impeller diameter and the ratio of hub diameter to tip diameter must be kept low. In that case, the outlet guide vane assembly for the or each impeller, whether it be the final outlet guide vane assembly 5: for a single or multiple stage machine or the intermediate guide vane assembly ix, is characterized by its geometric pitch decreasing in the direction from tip to hub. I 'he flnal outlet guide vanes 52: are, as is the usual practice in fans, helically oi the opposite hand to the 1mpeller blades 3 and their concavity is opposite to that of the impeller blades, bringing the discharging ends of the vane sections towards parallelism with the axis 1. The intermediate guide vanes 6a: are arranged to reverse the portion of drag remaining from the preceding impeller stage and thereby deliver the fluid to the next impeller stage 2a, 3a with counterswirl; the value of this counterswirl still being less than the drag of the impeller blades 3a. They are therefore characterized as regards helical hand by their receiving ends being of the opposite hand to, and their discharging ends being of the same hand as, the

chords of the impeller blades.

The Figure 4 formation is primarily one for the hub zone; and the tendency would always be for it to be modified progressively in the direction from hub to tip in such a manner as to approximate to the Figure 3 formation. Figures 3 and 4 may in fact be regarded as typical oi tip and hub formations respectively within the scope of the invention; and in general combinations of blades and vane assemblies according to the invention may include within their radial length any continuous range of cross sectional forms between extremes of which Figures 3 and 4 are typical.

The nature of the changes is as follows. As regards the inlet guide vanes l and the blades 3,

creases from the form 5 at the tip to infinity at some intermediate radial section and thereafter decreases but with reversal of helical hand and concavity until they reach form 5:: at the hub. In regard to the interstage guide vanes, the concavity, the helical hand of their chords, and the hand of the discharge ends 01' the sections remain unchanged from their form 8 at the tip in the change to the form 6x at the hub, but their receiving ends change in the same manner as do the final outlet guide vanes, so that the angle b decreases from some substantial value at the tip to zero at some intermediate radial section and then increases to some substantial value 01' opposite sign at the hub; accompanied by reversal of helical hand of the tangent l4. Two special cases within this range of change may be mentioned;

one is the case in which Figure 3 applies to the tip zone and the change 01 value of angle 1), along the length of the vane, stops at zero, so that at the hub tangent I4 is parallel to the axis 1. In

the second case the hub blade and vane formation is of the Figure 4 pattern and the change of the angle b reaches zero value at the tip.

Figures 9 and 10 show the invention embodied as a propelling device suitable for aircraft or ships, where it is useful inter alia in minimizing cavitation. As in the other examples the thrustproducing features of the blade sections increase in magnitude in the direction from tip towards hub and the geometric pitch of the blade de- 5 creases in the said direction. There is combined, with the propeller, an inlet guide vane assembly 4 which is helical and co-axial with the propeller and which is of decreasing geometric pitch to wards the axis. 10

In Figure 9 said vane assembly shadows only the hub zone of the propeller and so has the efiect or increasing the manometric efficiency only in that zone. It may be enclosed within a cylindrical casing l2; the cross section of which may 15 broad novelty is the combination of counterswirl,

not necessarily in excess, with an impeller of decreasing pitch towards the hub. From a commercial standpoint, the inventions may be compared by saying that the invention of application Serial No. 238,908 is for definite high pressures and that of the present invention is for pressures that are medium relatively to those oi! the first invention, although on the whole higher than those obtainable from the best modern fans. 35 Mechanical and volumetric efllciency are of greater importance in the present case than in the prior invention.

I claim:

1. A screw type fluid propelling apparatus com- 40 prising an impeller having blades of aerofoil cross-section increasing in width and in total area progressively in the direction from tip to hub, the geometric pitch of the blades progressively decreasing in the said direction, in combi-r 45 nation with fluid inlet means comprising a. helical guide vane assembly co-axial with the impeller, said assembly comprising guide vanes the axoncavity of which faces in the opposite circumferential direction to that or the rotor blades to 5m- 50 part counterswirl to the fluid ingoing to thee peller, said vane assembly being 01' decreasing geometric pitch in the direction from tip to hub.

2. A screw type fluid propelling apparatus comprising animpeller having blades oi. aeroioil 55 cross-section increasing in width, in total area, and in relative blade thickness progressively in the direction from tip to hub, the geometric pitch of the blades progressively decreasing in the said direction, in combination with fluid inlet means 9 comprising a helical guide vane assembly co-axial with the impeller, said assembly comprising guide vanes the concavity of which faces in the opposite circumferential direction to that of the rotor blades to impart counterswirl to the fluid 55 ingoing to the impeller, said vane assembly being of decreasing geometric pitch in the direction from tip to hub.

3. A screw type fluid propelling apparatus comprising an impeller having blades 01 aerofoil cross-section increasing in width and in total area progressively in the direction from tip to hub, the geometric pitch of the blades progressively decreasing in the said direction, in combina tion with fluid-inlet means comprising a helical 75 guide vane assembly co-axial with the. impeller, said assembly comprising guide vanes the con- F cavity of which faces in the opposite circumfercross-section increasing in width and in total ingoing to the impeller, a counterswirl, the mag-;

nltude of which is, along the whole length of the blade, substantially in excess of that 'of the impeller drag; said vane assembly being of decreasing geometric pitch in the direction froni tip to hub.

5. A screw type fluid propelling apparatus comprising an impeller having blades of aerofoil crosssection increasing in width, in total area, and in relative blade thickness progressively in the direction from tip to hub, the geometric pitch of the blades progressively decreasing in the said direction, in combination with fluid inlet means shadowing at least the hub zone of the impeller, said fluid inlet means comprising a helical guide vane assembly co-axial with the impeller, said assembly comprising guide vanes the concavity of which faces in the opposite circumferential direction to that of the rotor blades to impart counterswirl to the.fluid ingoing to the impeller, said vane assembly being of decreasing geometric pitch in the direction from tip to hub.

6. An encased screw type fluid propelling appatatus having blades of aerofoil cross-section in creasing in width and in total area progressively in the direction from tip to hub, the geometric 'pitch of the blades progressively decreasing in "the said direction, in combination with fluid inlet means comprising a helical guide vane assembly co-axial with the impeller, said assembly comprising guide vanes the concavity of which faces in the opposite circumferential direction to that of the rotor blades to impart counterswirl to the fluid ingoing to the impeller, the apparatus also comprising a helical co-axial outlet guide vane assembly, said guide vane assemblies being of decreasing geometric pitch in the direction from tip to hub. I

7. An encased screw type fluid propelling appa ratus having blades of aerofoil cross-section increasing in width and progressively in the direction from tip to hub, the geometric pitch of the blades progressively decreasing in the said direction, in combination with fluid inlet means comprising a helical guide vane assembly co-axial with the impeller, said assembly comprising guide vanes the concavity of which faces in the opposite circumferential direction to that of the rotor blades to impart to the fluid ingoing to the impeller, a counterswirl the magnitude of which is, at least in the tip zone, substantially in excess of that. of the impeller drag, the geometric pitch of said assembly "decreasing in the direction from tip to hub, the apparatus also comprising a hellcalco-axial outlet guide vane assembly, the concavity of which faces, at least in the tip zone, in the same circumferential direction to that of the impeller blades.

8., Apparatus as claimed in claim 7 in which the geometric pitch of the outlet guide vane assembly decreases in the direction from tip to hub.

9. Apparatus as claimed in claim 7 in which the inlet means shadows the whole area swept out by the impeller blades.

10. An incased screw type fluid propelling ap- I paratus having blades of aercfoil cross section increasing in width and in total area progressively in the directionfrom tip tohub, the geometric pitch of the blades progressively decreasing in the said direction, in combination with fluid inlet means comprising a helical guide vane assembly co-axial with the impeller, said assembly comprising guide vanes the concavity of which faces in the opposite circumferential direction to that of the rotor blades to impart to the fluid ingoing to the impeller, a counterswirl the magnitude of which is, at least in'the tip zone, substantially in excess of that of the impeller drag; said vane assembly being of decreasing geometric pitch in the direction'from tip to hub, the apparatus also comprising a helical and c0-axial final outlet guide vane assembly, the helical hand of which, at least in the tip zone, is the same as that of the impeller blades. 11. Apparatus as claimed in claim 10 in which at least in the tip zone, the tangents to the median lines of the vane sections at the receiving end of the outlet guide vane assembly make a substantial angle with the axis.

12. Multiple stage screw type 'fluidpropelling apparatus comprising a plurality of impellers, each impeller having blades of aerofoil crosssection increasing in width and in total area progressively in the direction from tip to hub, the geometric pitch of theblades' progressively decreasing in the said direction, the first impeller being associated with fluid inlet means comprising a helical guide vane assembly co-axial with the impeller, said assembly comprising guide vanes the concavity of which faces in the opposite circumferential direction to that of the rotor blades to impart to the fluid ingoing to the impeller, a counterswirl the magnitude of which is, at least in the tip zone, substantially in excess of that of the impeller drag; said vane assembly being of decreasing geometric pitch in the direction from tip to hub,,the apparatus also comprising between two impellers a helical and coaxial interstage guide vane assembly, the helical hand of which, at least in the tip zone, is the same as that of the impeller blades.

13. Apparatus as claimed in claim 12 comprising a final outlet guide vane assembly the geometric pitch of which decreases in the direction from tip to root.

14. Apparatus as claimed in claim 12 comprising a final outlet guide vane assembly the concavity of which faces, at least in the tip zone, with the concavity of the impeller blades.

15. Multiple stage screw type fluid propelling apparatus comprising a plurality of impellers each having blades of aerofoil cross section increasing in width and in total area progressively vane assembly co-axial with the impeller, said assembly comprising guide vanes the concavity 01' which faces in the opposite circumferential direc tion to that of the rotor blades to impart counterswirl to the fluid ingciug to the impeller, the geometric pitch of said guide vane assembly decreasing in the direction from tip to hub, and a helical co-axial interstagc guide vane assembly between two impellers, the receiving end of said assembly being, in the hub zone at least, ofthe opposite helical hand from that of the chord of the impeller blades and its discharging end being of the same helical hand as the chord of theimpeller blades.

16. Multiple stage screw type fluid propelling apparatus comprising a plurality of impellers, each having blades of aerofoil cross-section increasing in width and in total area progressively in the direction from tip to hub, the geometric pitch of the blades progressively decreasing in the said direction, the first impeller being associated with fluid inlet means comprising a helical guide vane assembly co-axial with the impeller, said assembly comprising guide vanes the concavity of which faces in the opposite circumferential direction to that of the rotor blades to impart to the fluid ingoing to the impeller, a counterswirl, the magnitude of which is in the tip zone substantially in excess of that of the impeller drag, said vane assembly being of decreasing geometric pitch in the direction from tip to hub, the apparatus also comprising between two impellers a helical and co-axial interstage guide vane assembly, the

helical hand of which, in the tip zone, is the same as that of the impeller blades, whilst in the hub zone, the receiving end of the interstage guide vane assembly is of the opposite helical hand from that of the chords of the impeller blades and its discharging end is of the same helical hand as the chords of the impeller blades, and comprising a flnal helical outlet guide vane assembly co-axia1 and associated with the last impeller, said outlet guide'vane assembly being, in the tip zone of the same helical hand as, and,

in the hub zone, of the opposite helical hand from that of the chords of the impeller blades and the concavity ot.the assembly in the hub zone being opposite to the concavity of the impeller blades, whilst its concavity in the tip zone' hub, in combination-with fluid inlet means comprising a helical guide vane assembly co-axial with the impeller, said assembly comprising guide vanes the concavity of which faces in the opposite circumferential direction to that of the rotor blades to impart counterswirl to the fluid ingoing to the impeller, said vane assembly being of decreasing geometric pitch in the direction from tip to hub.

18. A screw type fluid propelling apparatui comprising an impeller formed with blades, an inlet guide v'ane assembly associated with the impeller and an outlet guide vane assembly assoi ciated with the impeller; the concavity of the' vanes of the inlet guide vane assembly facing ,opposlte to the concavity of the impeller blades and the concavity oi the vanes of the outlet guide vane assembly facing, at least in the tip zone, with the concavity of said impeller blades; the geometric pitch of the impeller blades and of at least one of said guide vane assemblies decreasing in the direction from tip to hub.

19. A screw type fluid propelling apparatus comprising an impeller comprising bladesthe geometric pitch of which decreases in the direction from tip to hub and an outlet guide vane assembly associated with the impeller, the concavity of the vanes'ot said assembly facing, at least in the tip zone, with the concavity of the impeller blades.

20. Apparatus as claimed in claim 19 in which the outlet guide vane assembly is of a geometric pitch decreasing in the direction from tip to hub.

- JOHN TAYLOR MCINTYRE. 

